1. Field of the Invention
The present invention relates to optical transmission equipment with a ring structure where a plurality of nodes are connected in the form of a ring by transmission lines. More particularly, the invention relates to optical transmission equipment which employs a wavelength division multiplexing technique to transmit wavelengths multiplexed by wavelength division onto the working and protection transmission lines in both directions.
2. Description of the Related Art
In optical transmission equipment where a plurality of nodes are interconnected by optical transmission lines, there is known the structure where each node employs a service wavelength and a protection wavelength and where the wavelengths are multiplexed by optical wavelength division to transmit identical information signals in opposite directions. Conventional optical transmission equipment is constituted by optical fibers for interconnecting nodes, an optical signal wavelength transmission section for transmitting an optical signal of a wavelength, which corresponds to the information to be transmitted from each node, onto the optical fiber, and a wavelength selecting section for suitably selecting and receiving information (a desired wavelength) from optical signals of wavelengths.
In each node, a service wavelength is selectively received from optical signals being wavelength multiplexed and transmitted. For example, suppose that a service wavelength is represented by .lambda.s and a protection wavelength by .lambda.p. The optical signals with these wavelengths assigned thereto are transmitted through optical fibers in opposite directions. If an obstacle occurs, the node incapable of receiving the service wavelength will switch the service wavelength to the protection wavelength and receive transmitted information.
Each node is provided with a wavelength transmitting section and a wavelength selecting section. The wavelength transmitting section is employed when the node receives information from the optical fiber, and when the node transmits information, the wavelength transmitting section outputs both service wavelength .lambda.s and protection wavelength .lambda.p. That is, in the wavelength transmitting section, the wavelengths are multiplexed and transmitted onto the optical fiber. The same information signals are transmitted by the service wavelength and the protection wavelength, respectively.
The service wavelength and the protection wavelength are transmitted through the optical fiber in the aforementioned opposite directions, respectively. In a normal state, the wavelength selecting section of each node selectively receives the service wavelength .lambda.s from the signals being wavelength multiplexed and transmitted. In this way, communication is established between nodes.
When an obstacle occurs in the conventional optical transmission equipment, for example, when an obstacle occurs between nodes, the node downstream of the obstacle point with respect to the service wavelength .lambda.s becomes incapable of receiving information. In this case, at the aforementioned node, the wavelength to be selected is switched from the service wavelength .lambda.s to the protection wavelength .lambda.p by the wavelength selecting section.
As previously described, the information that is transmitted by the protection wavelength .lambda.p is the same as the information that is transmitted by the service wavelength .lambda.s, and only the directions in which the wavelengths are transmitted are opposite. Since the transmission directions are opposite, there are cases where a certain node is positioned upstream of the obstacle point with respect to the protection wavelength .lambda.p. Therefore, it is possible for the node to receive the protection wavelength .lambda.p. In this way, the communication is continued.
However, there are cases where for the service wavelength .lambda.s, a certain node is positioned downstream or upstream of the obstacle point with respect to the information that the node transmits to another node. In such cases it is impossible to receive the service wavelength .lambda.s with respect to the information that a downstream node transmits.
Therefore, in the node in which an obstacle occurs, the service wavelength .lambda.s and the protection wavelength .lambda.p are selectively used as usual with respect to the information that another node transmits. That is, with respect to the service wavelength .lambda.s, depending on whether or not a node can receive the information that another node transmits, each node selects the service wavelength .lambda.s when the information can be received, and selects the protection wavelength .lambda.p when the information cannot be received. In this way, communication can be continued even when an obstacle occurs on the transmission line.
In the conventional optical transmission equipment, as previously described, both the service wavelength .lambda.s and the protection wavelength .lambda.p are set to the wavelength that is transmitted from each node, and the number of nodes needs to be twice the number of wavelengths. The conventional optical transmission equipment, therefore, has the disadvantage that the wavelength band to be used has to be widened and that a great number of wavelengths have to be set to the wavelength selecting section.
Additionally, the conventional optical transmission equipment has the disadvantage that wavelength components and fibers cannot be effectively used, because identical information signals are transmitted in both a clockwise direction and a counterclockwise direction.